The U.S. Pat. Nos. 4,740,726 and 5,247,832 describe a vibratory resonator having an external tube and an internal oscillation rod. A piezoelectric element is secured in each case within a depression on the internal oscillation rod. The rod has a rectangular, elongated shape. The piezoelectric element exerts a bending moment on the rod that leads to wagging movements, given the details of the way in which the rod is embodied.
Disadvantageous in these patents are the special requirements that exist concerning the embodiment of the oscillation rod and the attachment of the piezoelectric element. Furthermore, the oscillation rod does not become excited directly to wagging oscillations, but rather to bending oscillations, which, as a result of the shape of the rod and the attachment of the piezoelectric element, lead to wagging oscillations.
The principle of oscillation-excitation in these patents is as follows: A piezoelectric element is attached to the oscillatable unit, or to a suitable diaphragm, e.g. using an adhesive. When voltage is applied to the piezoelectric element, it undergoes a change in length. Since the piezoelectric element is connected to the oscillatable unit, and the oscillatable unit does not experience an increase in length, the result is that the piezoelectric element and the oscillatable unit bend. With the application of an alternating voltage, a bending-oscillation consequently results. For logical reasons, it must be assured by special details of the oscillatable unit that there is a preferential direction for this oscillation. Furthermore, the piezoelectric element must be attached to the oscillatable unit such that the change in length is transferred essentially into the bending motion. At the same time, the connection must be made so that it still holds when in the bent condition. A preferred method of connection is by means of an adhesive. However, the use of most adhesives is possible only up to a certain temperature.
All in all, this method characterized by the detour through bending oscillations is thus very involved. It requires special measures in respect to the manner in which the piezoelectric element is attached, and in respect to the shape of the oscillatable element. Furthermore, the preferred method of attachment with adhesives is a problem in that such adhesives are applicable only up to a certain temperature. Moreover, it is in principle an unsatisfactory method in that the wagging movement of the oscillatable unit can be achieved only through the intermediate step of the bending oscillation.